Minority carrier semiconductor devices with improved stability

ABSTRACT

A method for improving the operating stability of compound semiconductor minority carrier devices and the devices created using this method are described. The method describes intentional introduction of impurities into the layers adjacent to the active region, which impurities act as a barrier to the degradation process, particularly undesired defect formation and propagation. A preferred embodiment of the present invention uses O doping of III-V optoelectronic devices during an epitaxial growth process to improve the operating reliability of the devices.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 08/463,371 filed on Jun.5, 1995, now abandoned.

BACKGROUND OF THE INVENTION

This invention is in the field of minority carrier semiconductordevices. The invention relates in particular to methods for improvingthe operating stability of light emitting semiconductor devices by theintentional introduction of impurities and the devices created usingthese methods.

Degradation of semiconductor minority carrier devices typically involvesan increase in the non-radiative recombination efficiency of the deviceduring the device's operation. The causes of this degradation depend onthe type of device, its structure, materials, and operating conditions.

A known double heterostructure light emitting diode ("LED") is shown inFIG. 1. LED 10 is comprised of an optically transparent GaPwindow/current spreading/contact layer 12, high bandgap AlInP upperconfining/injection layer 14, a lower bandgap (Al_(X) Ga_(1-X))₀.5 In₀.5P active layer 16, high bandgap AlInP lower confining/injection layer18, and conductive substrate 20, which may be formed from GaAs or GaP.P-type contact 21 and n-type contact 23 complete the LED. Lightextraction occurs through both the top surface and sides of the LED. Thelayers are generally doped so that the p-n junction is located near orwithin the active layer, and ohmic contacts 21 and 23 are made to thep-type and n-type regions of the device. The structure may be grown byany of a variety of methods including metal-organic chemical vapordeposition("MOCVD"), vapor phase epitaxy("VPE"), liquid phaseepitaxy("LEP"), molecular beam epitaxy("MBE"), and others.

FIG. 2 is an energy band diagram of the LED shown in FIG. 1. Whenforward biased, efficient injection of the minority carriers into activelayer 16 is achieved by careful placement of the p-n junction. Theminority carriers are confined within the active layer of the LED byhigh bandgap confining layers 14 and 18. The recombination processconsists of both radiative recombination which produces the desiredlight emission and non-radiative recombination, which does not producelight. Non-radiative recombination may result from crystal imperfectionswithin the LED as well as other causes. Light is extracted from the LEDthrough the LED's various transparent layers and surfaces and isfocussed into a usable pattern by various reflectors and lenses(notshown).

The LED illustrated in FIG. 1 is only one example of a minority carrierdevice. A variety of other minority carrier devices, including bipolartransistors, photodetectors, and solar cells operate on similar physicalprinciples. Semiconductor lasers often have a double heterostructure andsimilarly experience the competition between radiative and non-radiativerecombination. The performance and stability of all these devicesdepends upon maintaining a long carrier recombination lifetimethroughout the operating life of the device.

For the LED of FIG. 1, the output power is directly proportional to theinternal quantum efficiency and can be expressed as:

    η.sub.external ∝η.sub.internal ∝ 1+(τ.sub.r /τ.sub.nr)!.sup.-1,

where η_(internal) is the internal quantum efficiency, η_(external) isthe external quantum efficiency, τ_(r) is the radiative recombinationlifetime, and τ_(nr) is the non-radiative recombination lifetime. τ_(nr)is inversely related to the number of non-radiative recombinationcenters in the active region. The relationship η_(external) and theconcentration of non-radiative recombination centers is illustrated bythe graph shown in FIG. 3, which shows the external quantum efficiencyη_(external) decreasing as the concentration of non-radiativerecombination centers increases. A variety of crystal defects can act asnon-radiative recombination centers, including substitutional orinterstitial impurities such as Cr, Cu, Au, Fe, O and even such shallowdopants as Si, S, Se, native point defects such as self-interstitialsand vacancies, impurity or dopant related complexes and precipitates,surface and interface states, and dislocations and other extendeddefects. These defects can arise during the growth process due toincorporation of residual impurities or epitaxial defect formation.

A minority carrier device can degrade during operation for severalreasons. In an LED, the carrier injection efficiency or light extractionefficiency can change depending on the particular device structure andthe operating conditions. The most common cause of decreased deviceefficiency is an increase in the non-radiative recombination efficiencycaused by the formation of defects in the active region during stress.This process results in the gradual degradation of devicecharacteristics over time, as illustrated by the graph shown in FIG. 4.The graph shows that η_(external), the external quantum efficiency,decreases as the period of time the device is under stress increases.

A variety of physical processes contribute to the increase innon-radiative recombination centers in the active region during LEDoperation. Recombination enhanced or photo-enhanced defect reactionswithin the active region or at nearby edges or interfaces can contributeto the increase. Other processes include the diffusion or propagation ofimpurities, native point defects, dopants, and dislocations (also knownas dark line defects) into the active layer from other regions of thedevice. These defects and residual or unintentional impurities havealways been considered as detrimental to device performance and greatefforts have been expended trying to minimize the concentration of thesedefects and impurities.

SUMMARY OF THE INVENTION

A first preferred embodiment of the present invention comprises a methodfor improving the operating stability of minority carrier semiconductordevices by the intentional introduction of impurities into the layersadjacent to the active region, which impurities act as a barrier to thedegradation process, particularly undesired defect formation andpropagation. The semiconductor devices produced using this method alsocomprise this first embodiment of the invention. In a particular exampleof this first preferred embodiment, impurities are introduced byintentionally doping III-V optoelectronic semiconductor devices withoxygen("O") during an epitaxial growth step. Normally, O is consideredan efficient deep level trap, and undesirable in an optoelectronicdevice. However, as will be described in more detail below, using O inthe manner described herein improves device reliability without loss ofdevice efficiency.

The present invention will now be described in detail with reference tothe drawings listed and described below.

BRIEF DESCRIPTION OF THE ILLUSTRATIONS

FIG. 1 is a cross-sectional drawing of a known light emittingsemiconductor device;

FIG. 2 is an energy band diagram of the light emitting semiconductordevice shown in FIG. 1;

FIG. 3 is a graph of the concentration of non-radiative recombinationcenters versus the external quantum efficiency of the device shown inFIG. 1;

FIG. 4 is a graph of the stress time versus the external quantumefficiency of the device shown in FIG. 1;

FIG. 5 shows the effect of O in the active layer on η_(external) ;

FIG. 6 shows the effect of O in the p-type confining layer onΔη_(external) ;

FIG. 7 is a graph showing the concentration of oxygen in each of thelayers of a semiconductor device as taught by the first preferredembodiment of the present invention; and

FIG. 8 is a graph of the external quantum efficiency η_(external) ofdevices constructed according to the present invention versus stresstime.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

High efficiency visible LEDs can be made using an (Al_(X) Ga_(1-X))₀.5In₀.5 P material system. Such devices are structurally similar to theLED shown in FIG. 1. The substrate is typically either GaAs or GaP, theconfining layers are (Al_(X) Ga_(1-X))₀.5 In₀.5 P (0<X≦1), the activelayer is (Al_(Y) Ga_(1-Y))₀.5 In₀.5 P (0≦Y≦1) and the window layer is anoptically transparent and electrically conductive material such asAlGaAs or GaP. The most commonly used epitaxial growth technique forthese materials is MOCVD. In these materials, O incorporation occurseasily in the alloys containing Al and leads to undesirable deep leveldefects which cause efficient non-radiative recombination, resulting inlow initial η_(external). Several techniques are used to minimize Oincorporation in these alloys, including growth at high substratetemperatures, use of a substrate orientation which reduces Oincorporation efficiency, and growth with a high phosphorus overpressure(high V/III ratio).

The amount of O in the epitaxial structure has a major impact not onlyon the LED efficiency, but also on device reliability.

Experiments with independently varying the O concentration in each ofthe various layers of the epitaxial structure, using intentionallycontrolled O incorporation, with the O levels being kept low enough sothat the O doped layers remain conductive, showed that device efficiencydepends on the O concentration in the active region. However, devicereliability depends on the amount of O in the p-type confining layer.These results are shown in FIGS. 5 and 6. The graph in FIG. 5 indicatesthat η_(external) decreases with increasing O in the active layer. FIG.6 shows that as O in the p-type confining layer increases, degradationof the LED is reduced. The trend of decreasing η_(external) withincreasing O content in the active layer was expected, as O is known toform a deep trap which contributes to non-radiative recombination in the(Al_(X) Ga_(1-X))₀.5 In₀.5 P active region. However, the result ofimproved device stability with increasing O content in the p-type upperconfining layer was not expected. These results permit the simultaneousoptimization of the device's efficiency and reliability by correctlyadjusting the O profile of the epitaxial structure.

As the precise nature of the LED degradation mechanism is unknown, theprecise reason that this O doping improves device stability is alsounknown. The O may tie up or slow down the propagation of otherimpurities, native point defects, substitutional or interstitialdopants, or dislocations which would otherwise be free to propagate intothe active region from the confining layers, substrate, metal contacts,mismatched interfaces, edges, or epitaxial defects, causing devicedegradation.

In III-V semiconductor materials, O can be a deep level impurity, areactive impurity which may getter or passivate other impurities, or ashallow compensating impurity. The improvement in device reliabilitydescribed herein is due to one or more of these properties. Similarresults could be achieved by choosing other typically undesiredimpurities with similar properties. Other deep level impurities includethe transition metals, such as Cr, Fe, Co, Cu, Au, etc. Other reactiveimpurities which have gettering or passivating properties include H, C,S, Cl, and F. The choice of shallow compensating impurities depends onthe conduction type of semiconductor material. In a p-type region,shallow compensating impurities are shallow donors and in an n-typeregion, shallow compensating impurities are shallow acceptors. In p-typeIII-V semiconductors, shallow compensating impurities are the elementsin columns IVB and VIB of the periodic table, particularly the donors O,S, Se, Te, C, Si, Ge, and Sn.

A schematic diagram of the first preferred embodiment of the presentinvention is shown in FIG. 7. Some or all of the available methods tominimize O concentration in the structure are used, with emphasis onkeeping active layer 16 as O free as possible. In the case of MOCVD,techniques for reducing O include high growth temperature, high Poverpressure, proper substrate orientation (100) misoriented toward(111)A, for example!, source purity, reactor cleanliness, leakintegrity, etc.

In the case of (Al_(X) Ga_(1-X))₀.5 In₀.5 P LEDs, an O doping source isthen used to controllably introduce O into the p-type confining layer 14to improve reliability. Depending on the dominant degradation mechanismand device configuration, other layers may be doped in other devices.The O doping source could be O₂, H₂ O, alkoxide sources such as dimethylaluminummethoxide and/or diethyl aluminumethoxide, or other O bearingcompounds. Improved device stability occurs when the p-type confininglayer 14 is doped with O at a concentration of at least 1×10¹⁶ cm⁻³ andup to 5×10¹⁹ cm⁻³. Best results occur when p-type confining layer 14 isdoped with a concentration of O of about 1×10¹⁸ cm⁻³. The upper limit ofO doping in this material system is determined by theconducting/insulating transition in the confining layer. In otherdevices and material systems these ranges will of course vary. In someapplications it may be desirable to vary the O profile within the p-typeconfining layer.

The careful optimization of the O profile for the device shown in FIG. 7results in improved device reliability while maintaining high initialη_(external). This is illustrated by the graphs shown in FIG. 8 whichillustrate how higher concentrations of O in the p-type confining layer14 result in higher η_(external) after stress is applied for anincreasing period of time.

In this preferred embodiment, O is introduced as part of the epitaxialgrowth process. Other methods, such as implantation or diffusion, canalso be used.

Before the research which lead to the present invention, O doping ofIII-V semiconductors was only used for studying O related deep leveldefects and for growing semi-insulating materials. As stated previously,O in the active region has always been known to lower efficiency. Nopreviously known literature suggested the use of O doping into aconfining layer to improve device performance. The teachings of thepresent invention could further be used in the fabrication ofsemiconductor lasers, photodetectors, solar cells, bipolar junctiontransistors and other minority carrier semiconductor devices.

What is claimed is:
 1. A minority carrier semiconductor devicecomprising:an active region with a predefined minority carrierrecombination lifetime; at least one conductive region adjacent to theactive region, the adjacent conductive region being doped with oxygenand maintaining the predefined minority carrier recombination lifetimein the active region during device operation; an electrical contact; anda contact layer, interposing the adjacent conductive region and theelectrical contact.
 2. The minority carrier semiconductor device ofclaim 1 wherein the active region comprises a light emitting region of alight emitting diode and the adjacent region comprises an injectionregion of a light emitting diode.
 3. The minority carrier semiconductordevice of claim 1 wherein the active region comprises a light emittingregion of a double heterostructure light emitting diode and the adjacentregion comprises a confining layer of the double heterostructure lightemitting diode.
 4. The minority carrier semiconductor device of claim 3wherein the doping concentration of oxygen is between 1×10¹⁶ cm⁻³ and5×10¹⁹ cm⁻³.
 5. A light emitting semiconductor device comprising:asubstrate; a first confining layer overlying the substrate; an activeregion for generating light wherein the radiative recombinationefficiency is predefined overlying the first confining layer; a secondconfining layer, the second confining layer overlying the active region;window layer overlying the second confining layer; and electricalcontacts deposited on the substrate and the window layer, at least oneconfining layer doped with oxygen, the doped confining layer remainingconductive and decreasing the loss of radiative recombination efficiencythat the device experiences under operating stress.
 6. The lightemitting semiconductor device of claim 5 wherein the second confininglayer is p-type, and the doping concentration of the oxygen in thep-type confining layer is between 1×10¹⁶ cm⁻³ and 5×10¹⁹ cm⁻³.
 7. Thelight emitting semiconductor device of claim 6 wherein the dopingconcentration of the oxygen in the p-type confining layer isapproximately 1×10¹⁸ cm⁻³.
 8. In a minority carrier semiconductor devicecomprising at least an active region and at least one adjacent region, amethod for improving the reliability of the device comprising the stepof doping the at least one adjacent region with oxygen, the dopingdecreasing the degradation in performance that the device experiencesunder operating stress and leaving the adjacent region conductive. 9.The method of claim 8 wherein the active region comprises a lightemitting region of a light emitting diode and the adjacent regioncomprises an injection layer of a light emitting diode.
 10. The methodof claim 9 wherein the active region further comprises a doubleheterostructure light emitting diode and the adjacent region furthercomprises a confining layer of the double heterostructure light emittingdiode.
 11. The method of claim 10 wherein the doping concentration ofoxygen is between 1×10¹⁶ cm⁻³ and 5×10¹⁹ cm⁻³.